1. Field of the Invention
The present invention relates to a diagnosis device for diagnosing existence or nonexistence of an abnormality in an exhaust gas recirculation system that recirculates exhaust gas discharged to an exhaust system of an internal combustion engine into an intake system.
2. Description of Related Art
A known internal combustion engine has an exhaust gas recirculation system (EGR system) for recirculating part of exhaust gas to an intake system in order to improve emission characteristics. The exhaust gas recirculation system has an exhaust gas recirculation passage for connecting an exhaust system and the intake system of the engine with each other and an EGR valve for adjusting a flow passage area of the exhaust gas recirculation passage. An EGR quantity, or a quantity of the exhaust gas recirculated to the intake system through the exhaust gas recirculation passage, is controlled by manipulating an opening degree of the EGR valve. If the exhaust gas recirculation system recirculates part of the exhaust gas to the intake system, the exhaust gas reduces combustion temperature in a combustion chamber of the engine. Accordingly, generation of nitrogen oxides in the combustion chamber is suppressed and the emission characteristics are improved.
A proposed abnormality diagnosis device (for example, a diagnosis device described in JP-A-2002-256982) for diagnosing existence or nonexistence of an abnormality in the exhaust gas recirculation system measures a change in air intake quantity or a change in pressure inside an intake system while gradually changing the opening degree of the EGR valve when an operating state of the engine is stabilized. The diagnosis device determines that an abnormality exists if the change in the air intake quantity or the change in the pressure inside the intake system is less than a predetermined threshold value. The air intake quantity or the pressure inside the intake system changes due to the change in the opening degree of the EGR valve if the exhaust gas recirculation system is normal. By using these characteristics, the above-mentioned abnormality diagnosis device diagnoses the existence or nonexistence of the abnormality.
In recent years, an exhaust gas recirculation system having a cooling device for cooling the exhaust gas in the exhaust gas recirculation passage to increase the EGR quantity is proposed, specifically, as an exhaust gas recirculation system for a diesel engine, intending to reduce a concentration of the nitrogen oxides in the exhaust gas. In this exhaust gas recirculation system, a cooling water passage is provided next to a wall of the exhaust gas recirculation passage so that the heat of the high-temperature exhaust gas in the exhaust gas recirculation passage is absorbed by cooling water. Thus, the EGR quantity can be increased.
In such an exhaust gas recirculation system, there is a possibility that solid materials such as carbon contained in the exhaust gas will accumulate in the exhaust gas recirculation passage because the exhaust gas in the exhaust gas recirculation passage is cooled by the cooling device. There is a possibility that the accumulation quantity of the solid materials will increase and the cooling function will be significantly degraded while the exhaust gas recirculation system is used for a long time.
The degradation of the cooling performance of the cooling device will cause reduction of the EGR quantity. The reduction of the EGR quantity due to the degradation of the cooling performance cannot be compensated for if the EGR quantity is controlled by open-loop control. The reduction in the EGR quantity due to the degradation of the cooling performance of the cooling device can be inhibited by adjusting the opening degree of the EGR valve if the exhaust gas recirculation system feedback-controls the EGR quantity. However, also in this case, a sufficient quantity of the exhaust gas cannot be recirculated under an operating condition in which a large EGR quantity is required.